In various applications, it is necessary to bond diamond, polycrystalline diamond compacts, or other superabrasives such as polycrystalline cubic boron nitride to a metal or matrix support. An uncoated superabrasive can be mechanically bonded to a matrix by surrounding and partially encapsulating it in the matrix material. However, such mechanical bonding decreases the cutting efficiency of the tool due to the limited exposure of the cutting element, and is prone to loss of the cutting element as the matrix abrades away during use.
Diamonds and polycrystalline diamonds ("PCD"s) have been coated with carbide forming metals in order to assist bonding with supporting matrices by forming chemical bonds there between. For example, diamond has been coated with tungsten to facilitate bonding to a backing such as an infiltrated tungsten carbide matrix as is disclosed in U.S. application Ser. No. 095,054, filed Sept. 15, 1987, assigned to the present assignee and incorporated herein by reference. The coated PCD is typically placed in a mold with a powder or particles of the matrix material. Common matrix materials include, for example, tungsten carbide, cemented tungsten carbide, tungsten powder, iron powder, iron alloy powder or cast tungsten carbide, which is a eutectic of tungsten monocarbide (WC) and ditungsten carbide (W.sub.2 C). A binder infiltrant, usually a copper based alloy, is melted and infiltrated through the matrix powder at temperatures between 1000.degree.-1200.degree. C. Upon cooling, the infiltrant bonds the matrix particles together and also bonds to the metal coating of the PCD. The binder typically occupies 30-50% by volume of the matrix.
It has been found that many metal coated PCD's bonded to infiltrated matrices have exhibited cracking at the diamond-coating interface and within the body of the PCD. It is believed that this is caused by the stress between the PCD and the infiltrated matrix body which occurs during cooling as a result of the different thermal expansion rates of the PCD and the matrix. Such cracks are typically parallel to the PCD-matrix interface and in some cases may include said interface. These cracks can later intersect with cracks caused by the stresses of actual use, thereby resulting in the premature loss of the PCD in the field.
A micrograph of a prior art tungsten coated PCD bonded to a matrix support is shown in FIG. 1. In particular, FIG. 1 shows a GE 2164 polycrystalline diamond 31, produced by the General Electric Co. under the trademark "Geoset", having a CVD applied tungsten coating which is 10 microns thick. Cracking is particularly a problem with larger size porous temperature stable PCD's ("TSPCD's") such as the GE 2164. The TSPCD 31 was infiltration-bonded to a cast tungsten carbide matrix using a copper based infiltration alloy having a solidus temperature of 1650.degree. F. (900.degree. C.). A thermally induced crack 29, shown in FIG. 1, is illustrative of the type of cracking which can occur during cooling after a metal coated PCD is bonded to a matrix.
The differential shrinkage occurring during this cooling process has been estimated. A typical matrix consisting of cast tungsten carbide powder and a copper based alloy shrinks 10.times.10.sup.-3 in./in. in cooling to room temperature from a solidus of 1650.degree. F. The diamond shrinks an estimated 2.7.times.10.sup.-3 in./in. The difference between these is 7.3.times.10.sup.-3 in./in. or approximately 0.7%. Therefore, it is believed that differential shrinkage values of less than approximately 0.7% are required to prevent cracking upon cooling after the PCD has been bonded to the matrix. In accordance with the present invention, polycrystalline diamond and other superabrasive or wear-resistant materials are coated with multiple metal layers in order to prevent such cracking.